1. Field of the Invention
This invention relates generally to a dynamic braking load analyzer and, more particularly, to a dynamic braking load analyzer that selectively determines a braking resistive load that dissipates power from an induction motor that is decreasing in speed through a variable frequency drive or a servo-drive.
2. Discussion of the Related Art
Variable frequency drives (VFDs) are sometimes used with three-phase induction motors to provide speed control and energy savings for various industrial applications. For many of these applications, the motor may tend to over-speed when the motor is decelerated to a slower speed or completely brought to a stop. In this situation, the motor becomes a generator, where the generated voltage is directed back to the variable frequency drive. The variable frequency drive detects the generated voltage, and then acts to dissipate the power into some device, such as a resistor, or send it back to the original power source that provided the power for the motor. Sending it back to the original power source is typically an expensive option because it requires additional devices coupled to the power source for controlling the power.
A resistor used for this purpose is known as a dynamic braking load resistor. The size of the resistor required to dissipate the power generated from the deceleration of the motor would depend on how fast the motor was rotating, how fast the motor would be rotating at the decelerated speed and the time needed for reducing the speed of the motor.
Currently, the proper size of a dynamic braking load resistor is provided through selection charts and/or catalogues. If the desired criteria, such as voltage, horsepower, duty-cycle, torque, watts and ohms, are determined based on an VFD manual and the application using the drive, then the appropriate resistor may be selected. It is typically straightforward to determine the voltage and horsepower seen by the braking load resistor because these values are based on the drive and/or the motor. Thus, the challenge is to determine the torque, duty-cycle and watts of the motor so that the selected resistor is neither oversized, which has cost implications, nor undersized, which may cause the resistor to overheat and possibly fail.
Determining the values of torque, duty-cycle and watts is a function of knowing the application parameters. This determination includes all mechanical devices that are connected to the motor that provide mass, speed and velocity considerations. These load devices will both contribute and add to the mass and breaking power required, or will act to absorb the power dissipated.